1. Field of the Invention
This invention relates to a recording and playback apparatus using a helical-scan-type rotary head for recording code data, audio information, video information, and so forth, on a tape as a recording medium.
2. Description of the Related Art
There is known a rotary-head-type recording and playback apparatus using rotary heads for recording and reproducing digital audio data, digital or analog video data on and from a magnetic tape running along the cylindrical surface of a drum rotated at a predetermined speed. In recording, slant tracks are made successively on a magnetic tape. Upon playback, the heads scan these tracks. In order to increase the amount of recorded data by the guard-bandless approach, the azimuth recording system has been employed.
A narrow-track approach which diminishes track pitches would be another approach for increasing the amount of digital data on a given length of tape, i.e. on a cassette tape. The recorded pattern with narrow tracks is a new format with a different track pitch from those of old formats. It is necessary, in general, for a recording and playback apparatuses employing the new format to be available for recording and playback of the new format but also for playback of signals of the old format.
In case of the azimuth-recording system, the azimuth effect (the effect of suppressing the crosstalk from adjacent tracks caused by an azimuth loss) becomes large with an increase of the ratio of the bit length (the width of one bit of data taken in the lengthwise direction of the track) relative to the track pitch Tp on a tape (Tp/bit-length). This relation is explained in greater detail with reference to FIG. 1.
In FIG. 1, W represents the track width, Gr the gap line of the recording head, and Gp the gap line of the playback head. Gap lines Gr and Gp are not parallel but make an angle, .THETA.. In this case, since the distributed phase of the flux density Bs(x) on the surface of the tape (normal to the plane of the paper) is not uniform on the gap of the playback head, an amplitude loss occurs in the playback signal. The loss is called gap inclination loss and gap angle loss.
The loss can be calculated by dividing the track width W into small segmental widths dz and integrating the contribution from each segmental track in terms of W. In case of recording a sinusoidal wave, since Bs(x) distributes in the form of Bsm cos(2.pi.x/.lambda.), Bs at a point distant by z from the track center (x-axis) in the width direction is expressed as Bsm{2.pi.(x+z tan.THETA.)/.lambda.}. Therefore, the total flux .phi. entering into the gap line Gp of the playback head is expressed by the following equation. ##EQU1##
If the gap inclination loss L.THETA. is defined as the ratio of the amplitude BsmW of .phi. under .THETA.=0 relative to the amplitude of EQUATION (1) and is expressed in decibel, the following equation is introduced. ##EQU2##
As seen from EQUATION (2), when W/.lambda. is large, the gap inclination loss is large, and the crosstalk from adjacent tracks can be reduced; and when W/.lambda. is small, the gap inclination loss is small, and the crosstalk from adjacent tracks increases. Therefore, when the track pitch Tp (=W) is decreased by the narrow-track approach, the azimuth effect is weakened particularly in long wave regions. To compensate for it, rotary heads with large azimuth angles are used.
As discussed above, at least two rotary heads with azimuth angles for the old format and at least two rotary heads with larger azimuth angles for the new format are required. Moreover, in a data storage device, it is necessary to verify by RAW (read after write) whether the digital data has been recorded properly. Ordinarily, therefore, heads for common use in playback and RAW are provided, which results in the use of eight heads in a data storage apparatus.
Mounting such a number of heads on a rotary drum complicates the arrangement of the rotary heads mechanism and hence increases the cost. Additionally, more channels in rotary transformers and more changeover switches as needed, which are additional factors that complicate the arrangement and increase the cost.